Sergeant Justin Richey from Palestine, Texas, is watched by Iraqi soldiers as the 1st Battalion, 5th Cavalry Regiment of the 1st Cavalry Division and the 887th Engineer Support Company of the 101st Airborne, survey the future site of a Joint Security Station in west Baghdad, Iraq, on Tuesday. Combat engineers in Baghdad are busy building and fortifying new outposts to prepare for a large military effort to pacify the Iraqi capital.

BAGHDAD, Iraq — Four of the five Americans killed when a U.S. security company's helicopter crashed in a dangerous Sunni neighborhood in central Baghdad were shot execution style in the back the head, an Iraqi military official said today.

The official — who, like all others contacted, spoke on condition of anonymity because details had not been made public — said the four were shot while on the ground. He gave no other details.

A U.S. official in Baghdad, however, had earlier said there was no information to substantiate the report that the bodies had been shot execution style.

The senior Iraqi military official said the helicopter had been shot down, but this was disputed by a U.S. military official in Washington. The Iraqi said the helicopter was hit by a machine gunner over the Fadhil neighborhood on the east side of the Tigris River, while the U.S. official said there was no indication in initial reports that the aircraft, owned by Blackwater USA, had been shot down. The Americans said they did not know what caused the aircraft to crash.

Blackwater USA confirmed that five Americans employed by the North Carolina-based company as security professionals were killed. The statement from spokeswoman Anne Tyrrell did not provide identities or any details of the fighting.

On Wednesday, U.S. Ambassador Zalmay Khalilzad offered condolences for the five Americans killed, calling them good men and saying he had traveled with them.

"We had a very bad day yesterday," Khalilzad told reporters during a roundtable discussion at the embassy in the heavily fortified Green Zone in Baghdad. "We lost five fine men."

He said he had traveled with the men who were killed and had gone to the morgue to view the bodies, although he didn't say where that was.

Khalilzad did not give more details, saying the crash was still under investigation and it was difficult to know exactly what happened because of "the fog of war."

Another American official, who spoke on condition of anonymity, said three Blackwater helicopters were involved. One had landed for an unknown reason and one of the Blackwater employees was shot at that point, he said. That helicopter apparently was able to take off but a second one then crashed in the same area, he added without explaining the involvement of the third helicopter.

The Qatar-based Al-Jazeera television reported that the 1920 Revolution Brigades insurgent group claimed responsibility for shooting down the helicopter and showed a video taken by a cell phone of a mass of still-smoldering twisted metal that it was said was the wreckage of the chopper.

Another Sunni insurgent group, the Ansar al-Sunnah Army, also claimed responsibility for the attack.